### Session A3: Bilayer 2D Systems: Interlayer Drag and Spontaneous Coherence

 Monday, March 13, 2006 8:00AM - 8:36AM A3.00001: Coulomb drag experiments in dilute p-GaAs double layer systems Invited Speaker: Ravi Pillarisetty Low density (or dilute) two-dimensional systems, which have large ratios of Coulomb interaction energy to kinetic energy (r$_{s}$ is roughly greater than 10), are found to exhibit some bizarre transport properties. These include an anomalous metallic temperature dependence and an apparent metal-insulator transition. Furthermore, the application of an in-plane magnetic field, which spin polarizes the 2D system, produces some very unique effects, including a giant magnetoresistance. These unusual transport properties have raised serious doubts regarding the applicability of Fermi liquid theory to the large r$_{s}$ regime. Despite intense efforts, no conclusive understanding of these transport anomalies currently exists. To gain new insights into the role the strong carrier interactions play in this regime, we have measured the Coulomb drag in low density p-GaAs 2D bilayers. The drag resistivity is directly proportional to the interlayer carrier-carrier scattering rate, and allows us to directly study the carrier interactions in this regime. Our findings are that as the density is lowered into the large r$_{s}$ regime, the drag resistivity develops a two to three orders of magnitude enhancement over that expected from simple Fermi liquid calculations. We also observe significant deviations from the expected T$^{2}$ dependence, which correlate with the anomalous metallic temperature dependence observed in the single layer resistivity. Furthermore, we find that both the single layer resistivity and drag resistivity exhibit the exact same qualitative in-plane magnetic field dependence, with both exhibiting similar features associated with spin polarization. These observations suggest that the origin of these transport anomalies, affects both the single layer resistance and drag resistance in exactly the same way, and is surprising since these are two extremely different transport properties. We conclude by discussing these experimental results in light of recent theoretical interpretations of our data. Monday, March 13, 2006 8:36AM - 9:12AM A3.00002: Many-body effects in low-density strongly-interacting 2D structures: Fermi liquids or not? Invited Speaker: Sankar Das Sarma I will discuss our many-body theoretic studies of low-density semiconductor-based 2D carrier systems, emphasizing remarkable qualitative and semi-quantitative agreement with recent experimental measurements. In particular, our Fermi liquid many-body theory quantitatively explains the experimentally measured density, temperature, disorder, and magnetic field dependence of bilayer drag, thermodynamic parameters (e.g. spin susceptibility, quasiparticle effective mass), and transport properties in both electron and hole-based low-density 2D semiconductor structures. Our work convincingly demonstrates that the 2D electron liquid remains a Fermi liquid down to fairly low densities, and speculative non-Fermi liquid considerations are unnecessary in order to understand the phenomenology of low-density 2D systems. This work is done in collaboration with Euyheon Hwang, and supported by ONR, LPS, and NSF. Monday, March 13, 2006 9:12AM - 9:48AM A3.00003: Drag resistance of 2D electronic microemulsions Invited Speaker: Steven Kivelson In two dimensional electron systems with Coulomb or dipolar interactions, a direct transition, whether first or second order, from a liquid to a crystalline state is forbidden. As a result, between these phases there must be other (microemulsion) phases which can be viewed as a meso-scale mixture of the liquid and crystalline phases. We investigate the transport properties of these new electronic phases and present arguments that they are responsible for the various transport anomalies that have been seen in experiments on the strongly correlated 2DEG in high mobility semiconductor devices with low electron densities. In particular, motivated by recent experiments of Pillarisetty et al, PRL 90, 226801 (2003), we present a theory of drag in electronic double layers at low electron concentration. We show that the drag effect in such systems is anomalously large, it has unusual temperature and magnetic field dependences associated with the Pomeranchuk effect, and does not vanish at zero temperature. Monday, March 13, 2006 9:48AM - 10:24AM A3.00004: Transition between Composite-Bosons and Composite-Fermions in $\nu={{1} \over{2}} + {{1}\over{2}}$ Quantum Hall Bilayers Invited Speaker: Steven H. Simon There has been considerable recent interest in bilayer quantum Hall systems at filling fraction $\nu=1/2+1/2$. At large spacing between the layers, the system is described as two independent $\nu=1/2$ composite fermion fermi seas, with each electron being bound to two vortices of the wavefunction within the same layer. At small spacing between the two layers the system can be described as a composite boson condensate (also known as 111 state" or exciton condensate) where each electron is bound to one vortex of the wavefunction within the same layer and to one vortex of the wavefunction in the opposite layer. As the spacing between the layers is continuously decreased, intra-layer correlations must be replaced by inter-layer correlations, and the composite fermion sea must be replaced by the composite boson condensate. In this talk we will focus on the nature of this transition. For intermediate distances between the two layers, we propose a scenario where composite bosons and composite fermions coexist in two interpenetrating fluids[1]. In other words, we allow some electrons to bind to vortices within the same layer, and some to bind to vortices in the opposite layer. Trial wavefunctions describing these mixed composite-boson-composite-fermion states compare favorably with exact diagonalization results. A Chern-Simons transport theory is constructed that is compatible with experiment. More recent work[2] has shown that pairing interactions between the composite fermions occur. Once this pairing is treated properly we obtain almost perfect numerical agreement with exact diagonalizations. Possible implications for experiments are discussed.\\ \noindent [1] S. H. Simon, E. H. Rezayi, and M. V. Milovanovic. Phys. Rev. Lett. 91, 046803 (2003)\\ \noindent [2] G. Moller , E. H. Rezayi, and S. H. Simon, to be published. Monday, March 13, 2006 10:24AM - 11:00AM A3.00005: Spectroscopy of Emergent Phases of Electron Bilayers in the Quantum Hall Regime Invited Speaker: Vittorio Pellegrini Electron bilayers in semiconductor heterostructures in the quantum Hall regime are contemporary realizations of highly correlated systems where bizarre quantum phases may appear. A mean field configuration occurs when a tunneling gap splits the single-particle levels in their symmetric and anti-symmetric combinations and the Landau level filling factor is 1. This configuration has full spin and pseudospin ferromagnetic order, where pseudospin is a quantum operator describing layer occupation. The presentation considers optics experiments that offer evidence of the breakdown of the pseudospin order of the mean-field paradigm based on measurements of low-lying spin excitations [1]. The suppression of the pseudospin order manifests a new quantum phase that can be interpreted as a highly correlated fluid of electron-hole excitonic pairs across the tunneling gap. Evidence of a phase transition to a non-quantum-Hall phase is found in measurements of elastically-scattered light (Rayleigh scattering) and of spectral lineshapes of spin-excitations [2]. The transition occurs when the pseudospin order fully collapses by application of an in-plane magnetic field component. \newline \newline [1] S. Luin, et al. Phys. Rev. Lett. \textbf{94}, 146804 (2005). \newline [2] S. Luin, et al., to be submitted.